1. Field of the Invention
The present invention relates to image processing methods and systems for converting grey scale images into binary images and more particularly, to combining binary image slices with unique masks to improve the display of grey scale images on binary output devices.
2. Description of the Prior Art
Many data output devices display information in a binary format, i.e., the data is represented as black (off) or white (on) picture elements (pixels). Such devices include printers, computer displays, and facsimile machines. The display of binary data e.g. characters and lines, is compatible with the binary output devices and presents no problems in their display. A problem arises when it is necessary to display a grey scale image such as a photograph on a binary output device. A grey scale image contains continuous tone or halftone regions of various shades of grey rather than being black or white. Devices having the capability of displaying grey scale images can display at each pixel a range of intensity levels, usually from 0 to 256, which is normally determined by eight bits or one byte of computer memory. A binary output device can only display at each pixel one of two intensity levels which is determined by a single bit of memory per pixel. Thus, the difficulty is displaying the various shades of grey on a device having the capability of displaying only black or white pixels.
In order to display the grey scale image on a binary output device, the image must be converted to one bit per pixel. Several techniques have been developed in the prior art to perform this conversion. In one technique, known as thresholding, a binary video slice is made from the image based on the relationship of the intensity level of each pixel of the image to a predetermined threshold intensity level. A slice pixel will be turned on or made white if the value of the light intensity at a corresponding pixel location in the original image is above a certain value. Similarly, a slice pixel will be turned off or made black if the value of the light intensity at the corresponding pixel location is below the threshold value. This basic thresholding technique fails to satisfactorily display the continuous tones of the original grey scale image. U.S. Pat. No. 4,672,682 to Naruse et al. discloses a method for improving the basic threshold technique by successively increasing the slice threshold levels and determining the optimal threshold for displaying each pixel in a video scanning device to correct for temperature changes and other light reducing effects during the scanning operation.
Another method for converting grey scale images to binary is known as the dithering technique. In this technique, a group of pixels are selected to form a local grey spot corresponding to the intensity of an original grey scale image pixel. The number of pixels in the grey spot set to white are proportional to the grey scale value. Many strategies have been developed to improve the smoothness and sharpness of a dithered image. U.S. Pat. No. 4,517,606 to Yokomizo et al. is directed to a method which utilizes two separate dither patterns, one for characters and one for images with halftone. Different threshold levels are selected for each of the patterns. U.S. Pat. No. 4,709,274 to Tioka is directed to an improvement of the two dither pattern technique which utilizes AND and OR operations to improve the selection between the non-halftone areas and the halftone areas of the original image for producing the binary grey scale image. The generation or calculation of the dither patterns to approximate the original grey scale levels requires programs that take considerable time to run and are prone to several classic subjective errors such as worms, staircasing or quantitative residue.
Another technique for improving the dithering method is error diffusion in which the error in the approximation of the grey scale level by the calculation of grey spots can be spread over several neighboring pixels. Error diffusion blends regional grey scale and minimizes unnatural contouring, however, unwanted effects still result such as worms or harmonic patterns in display values. A description of the error diffusion technique can be found in an article by Floyd et al., Proc. S.I.D. Vol. 17(1976). Furthermore, a review of various methods for displaying grey scale images on binary devices can be found in an article by Jarvis, et al. CGIB 5(1976).
In addition, it is also necessary in certain applications to compress the original continuous tone image as it is displayed on the binary output device. Grey scale images are classically difficult to compress while binary images are more easily compressed as taught by Knowlton, IEEE Proc. 68(1980).